Amplifier circuit



' Sept. l2, 1944.

W. H. COOK AL AMPLIFIER CIRCUIT Filed July 15, 1943 er/ H 600A e /Venr)INVENTOR WMEM- ATTORNEYS.

tetrode or beam 110 grid. However,

- circuit consis Patented Sept. 12, 1944 Wilbert a. cook GuidedAMPLIFIER CIRCUIT New York, N. Y., and Henry C. Y Dalrymple, WestOrange, N.

Radio Corporation, corporation of New York J., assignors to New York, N.Y., 1!.

Application July 15, 1943, Serial No. 494,804

4 Claims;

to an amplifier circuit y to a circuit for operating a We! tube as azero bias class This invention relates and more particular] The objectsand advantages will be apparent from the following description and theaccompanying drawing which diagrammatically represents circuitconnections. embodying the invention.

Referring to the drawing in detail, there is a pair of tetrode tubes lconnected in a circuit for push-pull or balanced operation.- Thesetetrode tubes II) are of the type commonly known as beam power tubes, astandard tube of this type being commonly known to the trade as a 6L6tube. The elementsin such a tube are arranged in spaced relation andcomprise a heater l I, a cathode l2, a grid l3 adjacent said cathode, aplate l4 and a second grid Iiinterposed between the plate I 4 and thefirst-mentioned grid l3. In the ordinary operation of such a tube, thegrid l5 located between the plate l4 and the grid l3 normally functionsas a screen grid and the grid I 3 adjacent the cathode l 2 functions asa control in accordance with this invention, the grids [5, which arenormally the screen grids, in each of the tubes are respectively connected to opposite sides of a balanced input circult and function ascontrol grids. The input ts of-a transformer l6 which has its centertap'connected to ground at H. The cathodes H in each of the tubes andthe grids l3 adjacent thereto are connected together and then to groundat ill to establish a zero bias for these elements. The plates iii ofthe tubes are connected to an output plate voltage supply 13+. Theamplifier circuit just described .functions in general like a zero biasclass B amplifier but has many advantages thereover.

The grids l3 are relatively I2 and when connected to the cathodes, forma return path to the cathodes for a certain percentage of the electronswhich are in the normal space charge region near the-cathodes. Since thecontrol grids [5, which are: normally the screen grids, are connected toground through the input circuit, no acceleration of electrons occurs byvirtue .of these When the tubes are thus connected, a very low platecurrent will be drawn with plate voltage values up to approximatelytwice the voltages that would be applied to the tubes when operated in aconventional beam forming manner. With the-circuit connections shown,the operation is similar to close to the cathodes relatively greater notapplied toithe grids. In

transformer-I9 and to a s l that very .low values that of a zero biasclass B triode except that the spacing between the control athodes l2than in the triode lower input capacitance. Be-

grids l5 and the 0 results in much cause of this lowered inputcapacitance, it is possible to have much greater impedance in thecontrol grid circuits than is possible in an ordinary zero bias class Bamplifier.

The increased impedance results in greater independence between theplate current and the plate voltage when excitation is applied to thecontrol grid l5 and it is possible for the plate impedance to varyconsiderably withoutintroducing serious distortion or loss of power.Also, a much higher plate voltage can be applied to these tubes withoutexceeding the limits of normal plate dissipation than is possible whenthe tubes are operated as beam power tubes.

When the tubes are connected in the manner shown, there is practicallyno heat dissipated in the grid structures because a steady voltage isthe push-pull arpolarity of the grids reverses during each alternationof the input signal Thus, failures, due to improper grid alignment andemission resulting from high operating temperatures, are eliminated.

In practice, this circuit has been found to operate'with very littlepower input and with rangement shown, the

good overall efliciency has been obtained and the circuit possesses theadvantages of low mu class B triode operation without the necessity ofsupplying the critical bias voltage necessary in such operation withtriodes.

Tube life at double the plate voltage normally applied in beam operationhas been increased to three times the normal beam tube operating lifewith no measurable change in the tube characteristics. The poweroutput'obtained has been found to be somewhat greater than with the sametubes connected for beamoperation. The simplification in apparatus andpower supply equipment resulting from this type of operation isconsiderable.

Distortion measurements indicate that the discs of the tubes, which'areconnected for push-Pull operation, are substane linear even at lowlevelsof operation so grid impedances as high as 40,000 ohms. Very.

of distortion result at low levels as compared with an ordinary zerobias class B amplifier. The general operating characteristics as regardsstability and distortion are similar to those obtained in the operationof class A triodes.

While the invention has been shown and described in connection with apreferred embodiment, it is not to be limited to the exact form shownand described as it will be apparent to those skilled in the art thatcertain modifications and changes may be made without departing from thescope of the invention.

We claim:

1. A zero bias class B amplifier circuit including a pair of beam powertubes connected for push-pull operation, said tubes being tetrodes, eachhaving as elements a heater, a cathode, a grid adjacent said cathode, aplate and a second grid located between the first-mentioned grid and theplate, means for establishing a zero bias on the cathodes of said tubesand their adjacent grids, an input circuit connected to thesecond-mentioned grids and an output circuit connected to the plates.

2. A circuitfor operating a beam power tube as a zero bias class Bamplifier comprising a pair of beam power tubes, said tubes being of thetetrodetype, each having a heater, a cathode, a

grid adjacent the cathode, a plate and a second grid interposed betweenthe first-mentioned grid and the plate, a circuit connecting thecathodes and the grids adjacent thereto to ground, a balanced inputcircuit connected to the secondmentioned grids and an output circuitconnected to the plates.

3. An amplifier circuit including a pair of beam power tubes connectedfor push-pull oper- 10 ation, each of said'tubes having as elements aheater, a cathode, a grid adjacent said cathode, a plate, and a secondgrid located between the first grid and the plate, a balanced inputcircuit connected to the second grids, an output circuit is connected tothe plates, and a circuit connecting the first grids and the cathodes toground.

4. An amplifier circuit including a pair of beam power tubes connectedfor push-pull operation, each of said tubes having as elements a 20heater, a cathode, a grid adjacent said cathode,

a plate, and a second grid located between the first grid and the plate,a balanced input circuit connected to the second grids, an outputcircuit connected to the plates, and means for estab- 25 lishing a zerobias on the cathodes and their adjacent grids of each of the tubes.

' WILBER'I' H. COOK.

HENRY C. DALRYMPLE.

